Synchronous power systems are commonly used for efficiently powering electrical motors that drive fans, compressors, pumps, and other types of loads. Asynchronous electrical machines produce (e.g., motors) or consume (e.g., generators) torque only in conditions where the mechanical speed is different than the electrical speed. The magnitude of the difference of mechanical and electrical speeds is commonly referred to as “slip”. Asynchronous motors produce at least partial, and up to full rated, torque at all mechanical speeds less than the electrical voltage speeds, thus allowing acceleration rapidly to near matching electrical and mechanical speeds when connected to an electrical bus operating at constant speed, or “line start”. The ratio of electrical speed with respect to mechanical speed of an electrical machine is defined by the number of magnetic pole pairs of the specific design. Examples of asynchronous motors are induction motors which function based on Eddy current phenomena and hysteresis motors which rely on magnetic hysteresis phenomena. Induction motors are commonly used to drive mechanical loads from fixed speed national electric grids due to their “line start” capacity.
Synchronous electrical machines produce (e.g., motors) or consume (e.g., generators) torque only in conditions where the mechanical speed is equal to the electrical speed and the rotor and stator magnetic poles are misaligned. Synchronous machines commonly cannot “line start” due to the impractical requirement to connect the nonrotating motor to the rotating electrical grid at precisely aligned stator and rotor magnetic poles and develop sufficient torque to accelerate the rotor to electrical speed before misalignment exceeds ninety degrees electrical, where accelerating torque decreases and becomes negative at one hundred eighty degrees electrical.